I published another paper bearing the same title (but in French) in Scientia, 1918. The last sentences of that paper form the epigraph of the present one. As both papers are devoted to the explanation of the same fundamental ideas, there must needs be repetitions in the second one; I have not tried very hard to avoid them. Some changes and additions were suggested by my experience of the last five years. I ventilated them at a public lecture of the Boston meeting of the American Association for the Advancement of Science, in December 1922. The present explanation of my aims is divided as follows: I. Principles; II. The unity of knowledge; III. The unity of mankind; IV. The history of science; V. The New Humanism. To this are added two appenldixes; 1) The publication of Isis; 2) List of the first friends of Isis. The first appendix is, in fact, an urgent appeal to American scientists and scholars. Please do not fail to read it.